Turbine casing for an axial-throughflow gas turbine

ABSTRACT

The present invention relates to a turbine casing for an axial-throughflow gas turbine. The turbine casing surrounds at least one hot-gas space ( 5 ) between a compressor stage ( 7 ) and a turbine stage ( 6 ) and has an outer shell ( 1 ) as an external boundary and also an inner component ( 2 ) which is provided separately from the outer shell and which separates the hot-gas space from the outer shell via an annular space ( 3 ). The inner component ( 2 ) is connected to the outer shell ( 1 ) via two axial interfaces ( 4 ), in such a way that the annular space ( 3 ) is sealed off relative to the hot-gas space ( 5 ).  
     By virtue of this form of construction, the turbine casing withstands higher compressor final pressures and temperatures and can be produced cost-effectively.

[0001] The present invention relates to a turbine casing for an axial-throughflow gas turbine, said casing surrounding at least one hot-gas space between a compressor stage and a turbine stage and having an outer shell as an external boundary and also an inner component which separates the hot-gas space from the outer shell via an interspace.

[0002] In the case of axial-throughflow gas turbines, as a rule, the one or more compressor stages and the one or more turbine stages are arranged on a single shaft. The highly compressed and heated air flowing out of the compressor is supplied to a combustion chamber located within the turbine casing between the compressor stage and turbine stage. Due to the high pressure values and temperatures occurring in this region, the turbine casing is exposed to high load.

[0003] The development of high-compression compressors with rising compressor final temperatures leads to increasingly more stringent requirements as to the mechanical and thermal stability of the turbine casing. High-grade materials must constantly be found and used for the thermal and mechanical load which increases with a rising pressure ratio. At the same time, ever larger separating-flange screw connections of the turbine casing have to be provided, in order to withstand these loads. Both of these factors increases the cost of the plants considerably.

[0004] Another limiting factor is the manufacturing methods which are employed in the field of industrial gas turbines and in which the outer shells forming the turbine casing are cast. Due to the system employed, however, the mechanical and thermal load-bearing capacity of turbine casings produced by casting methods of this kind is limited.

[0005] The object of the present invention is to provide a turbine casing for an axial-throughflow gas turbine, which turbine casing can be produced cost-effectively and withstands very high pressures and temperatures. Thus, the turbine casing is to be capable of being operated without difficulty in the region of a compressor final pressure of more than 30 bar at temperatures of 550 to 570° C.

[0006] The object is achieved by means of the turbine casing as claimed in claim 1. Advantageous refinements of this casing are the subject-matter of the subclaims. The turbine casing according to the invention, which surrounds at least one hot-gas space between a compressor stage and a turbine stage and which has an outer shell as an external boundary, has an inner component which is provided separately from the outer shell and which separates the hot-gas space from the outer shell via an interspace. The inner component is connected to the outer shell via two axial interfaces, in such a way that the interspace is sealed off relative to the hot-gas space.

[0007] The turbine casing according to the invention is thus composed of an outer shell and of an inner component. By virtue of the arrangement of the two integral parts, the interspace formed between the inner component and the outer shell has a lower pressure and a lower temperature than the hot-gas space surrounded by the inner component. This is made possible, in particular, by the interspace being sealed off from the hot-gas space. A predeterminable pressure can be set in this interspace via suitable feeds to the latter.

[0008] By the turbine casing according to the invention being divided into an outer shell and an inner component, the thermal and mechanical loads occurring during operation are apportioned to the two components. In this case, the inner component, also designated hereafter as hot-gas component, is designed in such a way that it withstands both the circumferential stresses due to the pressure difference between the hot-gas space and interspace and the high temperature prevailing in the hot-gas space. This hot-gas component is therefore manufactured preferably from a high-grade material.

[0009] The outer shell must have only a sufficiently rigid design to be capable, on the one hand, of transmitting the static forces of the gas turbine and, on the other hand, of withstanding the pressure difference in the interspace and the ambient atmosphere. The temperature which acts on the outer shell is markedly reduced on account of the separation from the hot-gas space via the inner component and the interspace. This thermal load may be additionally counteracted by suitable cooling-air routing in the interspace formed between the inner component and the outer shell. This also reduces the phenomenon of the so-called “bowing” which is known in steam and gas turbines and is caused as a rule by deformation of the stator.

[0010] By virtue of the turbine casing being constructed according to the invention, it can be operated at compressor final pressures of more than 30 bar and the associated high temperatures. Due to the outer shell having to meet reduced requirements, the latter can be produced by means of conventional casting methods and simple materials, while high-grade materials are necessary only for the inner component exposed to the high temperature and pressure ranges.

[0011] In a highly advantageous embodiment of the turbine casing according to the invention, the inner component is connected to the outer shell by means of surface pressure acting in the axial direction. In this case, the outer shell has preferably two inwardly continuous projections or webs as axial interfaces onto which the inner component is placed. For this purpose, the inner component must have sufficient flexibility in the axial direction in order, over the entire operating cycle of the gas turbine, to build up at the axial interfaces with the outer casing sufficient surface pressure for the sealing effect which is to be achieved. The sealing effect is achieved preferably by a metallic sealing, both the axial interfaces and the surfaces of the inner component which come into contact with them having metallic sealing surfaces. The outer shell having the webs must, of course, have a sufficiently rigid design to absorb the axial forces occurring due to the surface pressure for metallic sealing. As a result of this refinement, the turbine casing according to the invention can be produced in a very simple way.

[0012] In a further refinement of the turbine casing, the materials for the outer shell and for the inner component are selected such that, during operation, there is sufficient surface pressure between the interfaces of the components for sealing-off purposes. The thermal longitudinal expansion coefficient of the material for the inner component is preferably selected lower than that for the outer shell. Different thermal expansions due to the different temperatures acting on the two components can thereby be compensated. The materials are, in every case, selected in such a way that the sealing effect between the inner component and the outer shell does not decrease during operation.

[0013] By a medium being suitably supplied under pressure into the interspace between the inner component and outer shell, for example, a pressure of 16 bar can be maintained in the interspace in the case of a pressure of 32 bar in the hot-gas space. In this case, the inner component and the outer shell only have to be capable in each case of withstanding a pressure difference of 16 bar.

[0014] The turbine casing according to the invention also makes it possible that, even under high pressure conditions of the compressor and with large diameters of the components, smaller separating-flange screw connections and simpler materials and geometries can be selected for the outer shell and the inner component. This, too, leads to a reduction in the costs for providing a turbine casing of this type.

[0015] A further advantage is the simple production of the casing, in which the inner component merely has to be clamped between the two axial interfaces. There is no need, in this case, for further connection techniques which could lead to thermal stresses or cracking.

[0016] The turbine casing according to the invention is explained further hereafter, without the general idea of the invention being restricted, by means of an exemplary embodiment, in conjunction with the drawings in which:

[0017]FIG. 1 shows diagrammatically a section through an exemplary turbine casing; and

[0018]FIG. 2 shows a perspective sectional view of the turbine casing from FIG. 1.

[0019] An example of a turbine casing for an axial-throughflow gas turbine is illustrated diagrammatically in FIG. 1. The figure shows, in this context, the upper part of the casing structure arranged symmetrically about a center axis 8. The center axis corresponds, here, to the gas turbine axis along which the shaft together with the turbine and compressor blades runs. The casing consists of the outer shell 1 and of the inner component 2. In the present instance, both surround the hot-gas space 5 annularly. The compressor stage 7 (not illustrated) is adjacent on the right side and the expansion space 6 with the turbine stage (not illustrated) is adjacent on the left side. The combustion chamber wall 9 is indicated (merely diagrammatically) in the hot-gas space 5. The combustion chamber may have any desired shape. In this case, both annular combustion chambers and multistage combustion chambers, such as are known from the prior art, may be provided. The hot-gas space 5 contains compressed air at high temperature, which has flowed in from the compressor stages 7, and also the hot gases escaping from the combustion chamber.

[0020] The hot-gas space 5 is surrounded by the inner component 2. Between the inner component 2 and the outer shell 1 is formed an annular space 3 which is sealed off from the hot-gas space 5 via the axial interfaces 4. The interspaces 4 are designed as metallic sealing surfaces, onto which the end faces of the inner component 2 press, so that surface pressure for metallic sealing is brought about. In this case, during assembly, the inner component 2 is clamped to the defined assembly gap between the two interfaces 4. In the transient operating range, during the start-up and the shutdown, an additional element (for example, a built-in diaphragm seal) assumes the sealing function. In the normal operating situation, the outer shell 1 and inner component 2 are braced relative to one another. In this case, the interfaces themselves are produced as radially continuous elevations or webs, the sealing surfaces of which run perpendicularly to the center axis 8. Both the outer shell 1 and the inner component 2 have an outwardly curved shape in this region. This shape is conducive to clamping the inner component 2 between the two axial interfaces 4.

[0021] The sealing off between the hot-gas space 5 and the annular space 3 allows markedly different pressure conditions in the annular space from those which prevail in the hot-gas space. The inner component 2 therefore has to support only the pressure difference between the hot-gas space and the annular space, while the outer shell 1 has to withstand only the pressure difference between the annular space 3 and the surroundings 10, that is to say atmospheric pressure, and also the static forces of the gas turbine. Furthermore, the separation of the outer shell 1 from the hot-gas space 5 via the inner component 2 and the annular space 3 lowers the thermal load on the outer shell 1, so that the latter can be manufactured from normally heat-resistant material.

[0022] Thus, the outer shell 1 may be manufactured, for example, from Stg41T, while the inner component 2 exposed to higher thermal loads is manufactured, for example, from the material Stg10T.

[0023] As regards conventionally designed turbine casings, the entire casing would have to be formed from the higher-grade material. In this case, too, a casing of this type in cast form would possibly not be capable of withstanding the high internal pressures.

[0024] In contrast to this, in the turbine casing according to the invention, only the inner component has to be formed from a high-grade heat-resistant material, while the outer shell can be cast in the conventional way. On the one hand, this reduces the costs and, on the other hand, this design withstands a higher compressor final pressure.

[0025]FIG. 2 shows the same exemplary embodiment again in a perspective sectional illustration. In this view, the curved shape of the outer shell 1 and of the inner component 2, together with the annular space 3 located between them, can be seen very clearly. The two axial interfaces 4, which are formed by continuous webs directed inward from the outer shell 1, are also evident. These interfaces 4 are manufactured preferably integrally with the outer shell.

[0026] The outer shell 1 of a turbine casing of this type can be produced very simply by means of a casting technique. The inner component 2 separating the hot-gas space 5 from the annular space 3 must then merely be clamped between the two interfaces 4.

[0027] Suitable material differences between the material of the inner component 2 and the material of the outer shell 1 makes it possible to exert a virtually temperature-independent surface pressure of the inner component 2 on the axial interfaces 4. The feeds for supplying a medium, for example a cooling medium, such as air, into the annular space 3 cannot be seen in the figure. A predeterminable pressure can be maintained in the annular space via these feeds.

[0028] List of Reference Symbols

[0029]1 Outer shell

[0030]2 Inner component

[0031]3 Annular space

[0032]4 Axial interface

[0033]5 Hot-gas space

[0034]6 Expansion space (turbine stage)

[0035]7 Compressor stage

[0036]8 Center axis

[0037]9 Combustion chamber wall

[0038]10 Surroundings 

1. A turbine casing for an axial-throughflow gas turbine, said casing surrounding at least one hot-gas space (5) between a compressor stage (7) and a turbine stage (6) and having an outer shell (1) as an external boundary and also an inner component (2) which separates the hot-gas space from the outer shell (1) via an annular space (3), the inner component (2) being connected to the outer shell (1) via two axial interfaces (4) in such a way that the annular space (3) is sealed off relative to the hot-gas space (5).
 2. The turbine casing as claimed in claim 1 , characterized in that the inner component (2) is clamped between the axial interfaces (4), so that the connection to the outer shell (1) is made by means of surface pressure acting in the axial direction.
 3. The turbine casing as claimed in claim 2 , characterized in that the outer shell (1) and the inner component (2) are formed from such different materials that, when the gas turbine is in operation, sufficient surface pressure is established at the axial interfaces (4) to seal off the annular space (3) relative to the hot-gas space (5).
 4. The turbine casing as claimed in one of claims 1 to 3 , characterized in that the axial interfaces (4) are designed as metallic sealing surfaces.
 5. The turbine casing as claimed in one of claims 1 to 4 , characterized in that the outer shell (1) and the inner component (2) surround the hot-gas space (5) annularly.
 6. The turbine casing as claimed in one of claims 1 to 5 , characterized in that the inner component (2) has an outwardly curved shape.
 7. The turbine casing as claimed in one of claims 1 to 6 , characterized in that the outer shell (1) has one or more orifices for supplying a medium to the annular space (3). 